B.1Raw Partition Overview

Datafiles for tablespaces can be stored on a file system or on raw partitions. A raw partition is a portion of a physical disk that is accessed at the lowest possible level. Input/output (I/O) to a raw partition offers approximately a 5% to 10% performance improvement over I/O to a partition with a file system on it.

To create a raw partition, you must first generate an extended partition and a logical partition. An extended partition points to raw space on the disk that can be assigned multiple logical partitions for database files. An extended partition also avoids the four-partition limit by letting you define large numbers of logical partitions to accommodate applications using the Oracle Database server. Logical partitions can then be given symbolic link names to free up drive letters.

B.1.1Physical Disk

A physical disk represents the entire disk and points to the following:

\Device\Harddiskx\Partition0

Symbolic link name \\.\PhysicalDrivex is automatically defined by Windows for every hard disk in the computer. For example, a computer with three hard disks will have the following symbolic links:

Partition0 is special, because it represents the entire physical disk regardless of any partitioning scheme on that disk. Windows writes a signature on the first block of all disks it recognizes. To avoid overwriting that block, Oracle Database skips the first block of a physical raw partition that is used for an Oracle Database datafile.

Note:

Although you can use physical disks, Oracle recommends that you use logical partitions.

B.1.2Logical Partition

Logical partitions point to drives other than \Device\Harddiskx\Partition0. They are initially assigned names with drive letters (\\.\drive_letter:) and typically re-assigned symbolic link names (\\.\symbolic link name). For example, \\.\D: may be assigned a symbolic link name of \\.\ACCOUNTING_1. Regardless of whether a drive letter or symbolic link name is used, logical partitions are defined to represent a specific partition in a disk rather than the entire disk. Internally, these names can expand to:

Oracle Database does not skip the first block of a logical raw partition used for an Oracle Database datafile.

B.1.3 Physical Disk and Logical Partition Considerations

Consider the following when deciding which raw partition to use:

Physical disks are automatically defined by Windows to represent the entire disk, and should never be defined by the user.

Logical partitions must be defined by the user to represent a specific partition in a disk. These partitions should be logical partitions or drives contained in an extended partition. They should never be defined as Partition0.

Using an entire disk (Partition0) for an Oracle Database datafile and using a partition that occupies the entire disk for an Oracle Database datafile are not the same thing. Even when a partition occupies the entire disk, there is still a small space on the disk that is not part of the partition.

If you are using an entire disk for an Oracle Database datafile (Partition0), then use the pre-defined physical raw names that Windows provides.

If you are using a specific partition and it occupies the entire disk, then use a logical partition.

If you are using a specific partition created with Windows disk-management tools, then define and use a symbolic link name rather than a logical partition number (even if it occupies the entire disk).

Note:

For both physical and logical raw conventions, use OCOPY to transfer the contents of a raw partition to a standard file system for backup purposes.

B.1.4 Compatibility Issues

You can create logical partitions, but define physical disk convention names for them. For example:

Oracle Database then handles datafiles using the physical disk convention even though it really is a logical partition. This will not cause any data corruption or loss as long as you continue to use physical disk naming conventions. Oracle recommends that you convert to the logical partition at your earliest convenience.

You can also create logical names representing Partition0, but this is definitely not recommended. For example:

\\.\ACCOUNTING_1 = \Device\Harddisk1\Partition0

This poses severe problems, because Disk Management typically writes a signature into the first block of every disk, and consequently may overwrite a portion of the datafile header. It can also cause data loss. Never use Partition0 with the logical partition convention.

Physical and logical partition conventions are not compatible with one another because of the extra block that is skipped for physical raw conventions. This also means you cannot simply use OCOPY to copy from a physical disk to a logical partition, because contents of these partitions are incompatible.

To convert from a physical convention to a logical convention, you must:

Perform a full database export to a (local) file system.

Create logical partitions and define logical names for these partitions.